1. Field of the Invention
The invention relates to the prevention of scorching, prior to vulcanization, of peroxide curable ethylene polymer based compositions.
2. Description of the Prior Art
Insulation compositions which are employed on electrical wire and cable are, in many cases, prepared from compositions which are based on vulcanizable, or cross-linkable, ethylene polymers. These ethylene polymer based compositions may be vulcanized, or cured, or crosslinked, with various organic peroxide compounds, as disclosed for example in U.S. Pat. Nos. 2,826,570; 2,888,424; 2,916,481; 3,079,370 and 3,296,189.
In the organic peroxide compounds which have been used to date for commercial purposes in these vulcanizable ethylene polymer based compositions, each oxygen atom in the peroxide group, i.e., --O--O--, of such compounds is directly attached to a carbon atom of an organic radical. The commercially useful compositions do not employ hydroperoxide compounds therein as curing agents because they have relatively high decomposition temperatures, and the free radicals provided by the decomposed hydroperoxides are not effective for cross-linking ethylene polymers.
In order to process the organic peroxide containing compositions so as to adapt them to be placed, as insulation, on the electrical conductor components of the wire and cable it is usually necessary to admix the components of the compositions at high temperatures, and to extrude them, again at high temperatures, onto the electrical conductor. These processing activities occur prior to the intended vulcanization of the peroxide containing compositions, which is usually accomplished after such compositions are extruded onto the electrical conductor.
It has been found, however, that when certain of the organic peroxide compounds, such as dicumyl peroxide, are used in combination with certain types of ethylene polymers or in certain types of ethylene polymer based compositions, that the entire curable composition is susceptible to scorching during the high temperature processing thereof prior to the vulcanization of the composition on the electrical conductor. Scorching is, in effect, the premature vulcanization of the insulation composition. This premature vulcanization usually occurs, when it occurs, in the barrel or die head of the extruder in which the insulation composition is being processed, at elevated temperatures, prior to its being extruded onto an electrical conductor, and prior to its intended vulcanization. When an insulation composition is scorched in the extruder, the extruded composition will have imperfections in the form of discontinuity and roughness in the surface of the extrudate; and lumps or surface ripples caused by gel particles in the body of the extrudate. In addition, excessive scorching may cause enough of a pressure build-up in the extrusion device to require a cessation of the extrusion operation entirely.
The tendency of a composition to experience scorch is a relative matter, since any vulcanizable ethylene polymer based composition can be made to scorch if processed under conditions designed to produce such result. Under a given set of conditions some compositions are more prone to scorching than are others.
Compositions which have been found to be more susceptible to scorching under a given set of conditions are those in which the ethylene polymer has a relatively low melt index and/or a relatively narrow molecular weight distribution.
The tendency of a composition to scorch under commercial operating conditions may be measured by means of the Monsanto Rheometer Test Procedure. The Monsanto Rheometer Test Procedure is described in ASTM-D-2084-71T.
Prior to the work of the present inventor as disclosed in this patent application, and three others filed on even date herewith, scorch prevention has been accomplished by the use of additives such as nitrites as disclosed in U.S. Pat. No. 3,202,648; the specific antioxidants and vulcanization accelerators disclosed in U.S. Pat. No. 3,335,124; and the chain transfer agents disclosed in U.S. Pat. No. 3,578,647. A mixture of two specific peroxides has also been used to provide a rate of cure that is intermediate the rate of cure of either of such peroxides, as disclosed in U.S. Pat. No. 3,661,877.